Fixing your new CJ7 Speedometer gauges

One of the issues I have found in restoring CJ7’s is the electrical.  There are so many more little whizbangs and doodads than in the early flat fender Jeeps.  The most common issue I have encountered has to do with the temperature and fuel gauges.  Working on a current CJ7 project I finished the wiring and found that both the fuel and temperature gauge were pegging all the way to Full and Hot.  This can happen on the old gauges when a resistor blows out and it can fry the gauges. Obviously something was wrong and to save some time I asked the customer to just order a whole new speedometer cluster.

That was a bad idea…. hopefully what I write here will save you some time should you find the need to replace your gauges or speedometer in your CJ7 (and I assume there can be similar issues in Cj5’s)

First off, here is the spiffy looking new gauge:

20161226_1658371

And note their commitment to Jeeps on the side of the box:

20161226_1659151

With how nice and complete the gauge looks and all the nice packaging you might figure,”hey, this is great!  Let’s install this and everything will work well and be happy again!”  That’s what my mechanics thought when I went away for Thanksgiving and they decided to help me out by installing this gauge for me.

When I came back from visiting family for Thanksgiving and found the gauge was installed I was all excited that this Jeep could go home.  I did not foresee this turning into the engineering adventure it became.

I started the Jeep and took it for a test drive.  The dash lights did not work anymore, neither the temperature gauge or fuel gauge registered anything, the brake warning light was dead…. all I could think was what the heck happened?  First I pulled the Jeep back into the shop, loosened the dashboard and checked the wiring.  The gauge was wired correctly.  Having run into other bad after market parts I pulled the gauge to check it.

Problem 1)  This is yet another part I received with an internal short.  From what I understand both main parts companies have issues with their gauges so check for shorts before even installing the gauge.

20161226_1705211

First problem I found using my voltmeter was a short in the fuel gauge.  The ignition terminal (I) showed zero ohms resistance when I connected the other test wire to the case. No resistance means a short.  12 volts straight to the body of the Jeep… not good.  This short that not only blew the gauge fuse, but also fried our new fuel sending unit I had just installed.  I opened the case of the new speedo, removed the fuel gauge, wrapped it in electrical tape, reinstalled it and now the short was gone.  The fuse stopped blowing and all lights kept working.

Problem 2)

Next I reconnected all the wiring, and immediately found that the fuel gauge and temperature gauge pegging all the way to Full and Hot again…. but why?  This is a new gauge!! Shouldn’t it work????

Well, the answer is no. NO.  Reading forum after forum I could not find a single person who had installed a new speedometer cluster and had the gauges work.  This disgusts me.  Apparently the manufacturer is fine with making and selling something that does not work, and the suppliers are fine with selling this garbage.  As long as things are being sold apparently nobody cares about the guy or girl in their garage going nuts because the new parts doesn’t work.

So I set about making this right.  I did alot of research and testing, and consulting with my retired engineer father, and we found a way to make these gauges work.

20161226_1641541

The original gauges were designed with resistors in them.  The resistors dropped the voltage down to about six volts for the gauges.  When the gauges keep pegging all the way to the top its because their resistors are blown on the original ones.  On the new gauges the problem is that THEY HAVE NO RESISTORS.

Old gauge:

20161226_1645531

20161226_1149121

New gauge:

20161226_213336[1].jpg

First off your new gauge will have a wire going from A on the gas gauge to A on the temperature gauge.  Remove that.  We will wire these gauges separately now.

Second, go to Radio Shack or order resistors online.  At Radio Shack I was able to pick up packs of 100 ohm 1 watt resistors (the small ones in the photo below) and 50 Ohm 10 watt resistors (big white one)

20161226_2104591

Just to show you how to check resistance take a look at the following image, and the settings on the voltmeter:

20161226_1650511

Third, you will wire the four 100 ohm 1 watt resistors side by side (in parallel is the technical term) and connect them to terminal I on the gas gauge.  You will connect the 50 Ohm 10 watt to terminal I on the temperature gauge.

Fourth you will split the 12 volt ignition wire that normally connects to I on the gas gauge.  Solder or otherwise connect two wires to the end terminal of the ignition wire, and run a wire to the four 10 ohm resistors, and another wire to the 50 ohm resistor.  This will provide proper resistance and make the gauges measure fuel and temperature correctly. My father designed a small plexiglass pane to make a secure attachment for all the wires.

20161226_2205271

20161222_1618231

20161226_1648211

I will add more images as I refine this process, but you must add these resistors for at least this version of the aftermarket gauge to work.  This is all setup to make the Omix gauge work.  I have not tested a Crown gauge yet but I understand that they do not function correctly either.

For information on proper resistance for the gauges here is one of a few good web pages:

CJ Gauge Specification Chart